The compressor wheel is the life-limiting component in turbochargers currently produced for commercial diesel engines. Changing the wheel material from aluminum to titanium alloy is one technical solution. However, compressor wheels have highly complex shapes and must be manufactured with high dimensional accuracy. The difficulty of working with titanium has inhibited the adoption of titanium compressor wheels in automotive air boost devices. The invention provides an economical process for the manufacture of titanium compressor wheels.
Air boost devices (turbochargers, superchargers, electric compressors, etc.) are used to increase combustion air throughput and density, thereby increasing power and responsiveness of internal combustion engines.
The blades of a compressor wheel have a highly complex shape which is design-optimized for (a) drawing air in axially, (b) accelerating this air centrifugally, and (c) discharging air radially outward with elevated energy (velocity/pressure) into the volute-shaped chamber of a compressor housing. In order to accomplish these three distinct functions with maximum efficiently and minimum turbulence, the blades can be said to have three separate regions.
First, the leading edge of the blade can be described as a sharp pitch helix, adapted for scooping air in and moving air axially. Considering only the leading edge of the blade, the cantilevered or outboard tip travels faster than the part closest to the hub, and is generally provided with an even greater pitch angle than the part closest to the hub (see FIG. 1). Thus, the angle of attack of the leading edge of the blade undergoes a twist from lower pitch near the hub to a higher pitch at the outer tip of the leading edge. Further, the leading edge of the blade generally is bowed, and is not planar. Further yet, the leading edge of the blade generally has a xe2x80x9cdipxe2x80x9d near the hub and a xe2x80x9crisen xe2x80x9d or convexity along the outer third of the blade tip. These design features are all engineered to enhance the function of drawing air in axially.
Next, in the second or transitional region of the blades, the blades are curved in a manner to change the direction of the airflow from axial to radial, and at the same time to rapidly spin the air centrifugally and accelerate the air to a high velocity, so that when diffused in a volute chamber after leaving the impeller the energy is recovered in the form of increased pressure. Air is trapped in airflow channels defined between the blades, as well as between the inner wall of the compressor wheel housing and the radially enlarged disc-like portion of the hub which defines a floor space, the housing-floor spacing narrowing in the direction of air flow.
Finally, in the third region, the blades terminate in a trailing edge, which is designed for propelling air radially out of the compressor wheel. The design of this blade trailing edge is generally complex, provided with (a) a pitch, (b) an angle offset from radial, and/or (c) a back taper or back sweep (which, together with the forward sweep at the leading edge, provides the blade with an overall xe2x80x9cSxe2x80x9d shape). Air induced and expelled in this way produces not only high flow, but also efficiently generates high pressure when diffused into a collecting duct or scroll.
Accordingly, functional considerations dictate the complex shape of a compressor wheel. The compound and highly complex curvatures of a turbocharger compressor wheel are most advantageously and economically obtained by a casting process wherein the wheel hub section and blades are integrally formed desirably from a lightweight material, such as aluminum or aluminum alloy chosen for its relatively low rotational inertia for achieving the further advantage of rapid accelerative response during transient operating conditions.
Recently, tighter regulation of engine exhaust emissions has led to an interest in even higher pressure ratio boosting devices. Aluminum compressor wheels are however not capable of withstanding repeated exposure to higher pressure ratios ( greater than 3.8), and have a relatively short, finite fatigue life. When a compressor wheel is rotated at operating tip speeds of 500 m/s or more, cast aluminum is subjected to relatively high tensile loading particularly in the hub region of the wheel which must support the wheel mass. Unfortunately, the hub region of any cast wheel is a site of metallurgical imperfections, such as dross, inclusions, and voids, which inherently result from the casting process. The presence of these imperfections in the vicinity of the central bore, which acts as a stress riser, renders the wheel highly susceptible to fatigue fracture in the hub region.
Accordingly, while economical to manufacture, cast compressor wheels are liable to failure. Failure of a compressor wheel necessitates at least replacement of the turbocharger, and may even cause damage to a vehicle engine. Thus, there is a need for a compressor wheel manufactured by a technique other than casting.
It is known that fatigue failures in compressor wheels can be significantly reduced by machining the compressor wheel from raw stock material, thereby avoiding the internal imperfections inherently resulting from a casting process. However, the complex machining requirements to form the impeller blades with the desired aerodynamic contours from wrought aluminum renders such a method for manufacture of aluminum compressor wheels impractical from a cost standpoint.
Titanium is much more difficult to work than aluminum, and material removal rates are low. Accordingly, the machining of titanium compressor wheels from wrought titaniumxe2x80x94generally beginning with a billet or form in the shape of a bellxe2x80x94is out of the question due to both high cost and amount of time required to produce the final net shape.
U.S. Pat. No. 4,850,802 (Pankratz et al) entitled xe2x80x9cComposite compressor wheel for turbochargersxe2x80x9d attempts to side-step the flaws inherent in casting, and teaches a composite compressor wheel comprising a cast shell and a noncast hub insert. The cast compressor wheel shell is formed from relatively lightweight, low inertia material, such as aluminum or, a selected aluminum alloy, and includes aerodynamically contoured impeller blades and a hub section having a recess in the base. A hub insert of a non-cast material, e.g., billet, is secured into this recess, and is sized and shaped to occupy regions within the compressor wheel subjected to relatively high stress during operation. Since the hub insert substantially occupies high stress regions within the wheel, wheel fatigue life is improved.
The above technique has been applied to the manufacture of hybrid compressor wheels for gas turbine engines. See U.S. Pat. No. 4,335,997 (Ewig) entitled xe2x80x9cStress resistant hybrid radial turbine wheelxe2x80x9d teaching a turbine rotor with radially extending blades for a gas turbine engine, wherein the hub may be forged titanium alloy, and wherein a shell of cast titanium alloy may be HIP bonded thereto to form a compressor wheel. This technique has, in practice, not even proven itself practical in the manufacture of gas turbine engine turbines, and certainly is much too costly and time consuming to be applied to mass production of small compressor wheels as employed in the automotive industry. Further, this technique requires separate manufacture then joining of two separate parts, and the integrity of the bond between the two parts is questionable.
There is thus a need for a simple and economical process for mass producing titanium compressor wheels, which process avoids the dimensional and structural imperfections such as dross, voids, and inclusions which inherently occur during a casting process, and which process also avoids the high cost associated with machining of titanium from blank. The process must be capable of reliably producing compressor wheels with high dimensional accuracy.
The present inventor investigated the above-described technical problems in the manufacture of titanium compressor wheels, and set out to develop a process by which each individual compressor wheel can be manufactured rapidly, economically, and yet with a very high degree of dimensional accuracy.
It is known to forge titanium. However, considering the complex shape of a compressor wheel as discussed above, with undercut recesses and/or back tapers created by the twist of the individual air foils with compound curves, not to mention dips and humps along the leading edge of the blade, one quickly reaches the conclusion that these net shape forging techniques have no relevancy in the manufacture of compressor wheels.
Considering also that the present invention is driven by economics, it follows that fewer process steps are better than more process steps, and that a single manufacturing technique involving only one type of equipment would be more economical than a single technique alone.
Departing from conventional wisdom, the present inventor attempted to combine two completely different techniques into a xe2x80x9chybridxe2x80x9d process. The present inventor attempted to first forge a titanium compressor wheel to a near net shape, and then finish machine the forged shape to produce the desired complex xe2x80x9cnetxe2x80x9d product.
It must be noted that it could not be predicted that a titanium product produced by a first rough forging step would be sufficiently reproducibly dimensioned so as to be able to be subject to the second process step of the present invention, namely, to be presented and indexed for finish machining by a xe2x80x9cblindxe2x80x9d tool in a fully automated process to produce a net shape product free of distortion and defect. That is, there would be no way to predict whether a rough forged part produced in an assembly line process could be positioned and oriented so accurately in, e.g., numerically-controlled cutting equipment that a thin layer of material could be machined from each blade surface.
Even more significantly, the overriding concern of the present invention was to produce an economical process for forming a titanium compressor wheel, and it would seem that a multi-step hybrid process would not be economical.
Quite surprisingly, despite requiring the application of two different types of manufacturing techniques, the inventive hybrid process made it possible to produce titanium compressor wheels with high accuracy, minimal imperfections as compared to cast products, and with great economy as compared to machining from wrought metal techniques. Further, the hybrid process lends itself readily to automation. A near net shape compressor wheel is first produced preferably by an automated process in a forging die, the near net shape approximating the final shape only to the extent possible with pullable forging dies, i.e., with xe2x80x9cundercutxe2x80x9d or xe2x80x9cbacksweepxe2x80x9d areas being xe2x80x9cfilled inxe2x80x9d to the extent necessary to prevent xe2x80x9cback-lockxe2x80x9d of the die inserts. This near net shape compressor wheel is referred herein to as xe2x80x9cnear net shapexe2x80x9d since only the xe2x80x9cundercutxe2x80x9d or xe2x80x9cbacksweepxe2x80x9d areas, which are filled in as discussed above, need to be machined in the subsequent machining step.
The forged titanium product, having the near net shape, can be machined by conventional techniques to remove the material needed to complete the backsweep and undercut areas of the blades. Problems associated with casting, such as mold-metal reaction, shrinkage, porosity, inclusions, etc., for which titanium is notorious, are eliminated by the process according to the present invention. The machined net shape has a high degree of dimensional trueness as compared to a compressor wheel cast from a wax pattern. Accordingly, this automated embodiment results in a highly accurate manufacturing technique.
Further yet, it is known that machining of titanium compressor wheels from stock titanium is expensive due to the amount of time required to machine away material (it may take an entire day to machine one titanium compressor wheel from stock) and due to tool wear (the greater the amount of material to be removed, the greater the amount of wear). In accordance with the present invention, since the amount of material being machined away is substantially less than in the case of machining from stock, the tool time and costs are negligible. When carried out on an industrial scale, since the amount of material to be machined in the machining step is small compared to the known technique of, e.g., manufacturing titanium compressor wheels from solid titanium stock using computer-aided manufacturing (CAM) equipment, the process of the present invention is surprisingly economical.
More specifically, according to the present invention, a titanium compressor wheel is easily and economically produced in an automated process using a first forging step. In accordance with the invention, xe2x80x9cundercutxe2x80x9d or xe2x80x9cbacksweptxe2x80x9d areas of the compressor wheel blades, or areas of twist, which would have produced a xe2x80x9cback-lockxe2x80x9dxe2x80x94preventing extraction of the forging die insertsxe2x80x94are filled in, but only to the extent necessary to make the forging inserts pullable. The term xe2x80x9cback-lockxe2x80x9d is conventional in the art as evidenced by U.S. Pat. No. 4,139,046 (Stanciu) entitled xe2x80x9cTurbine Wheel Pattern and Method of Making Samexe2x80x9d.
The forging process cannot by itself produce the desired final or net compressor wheel shape, since filled in areas need to be removed by machining. The forged wheel is thus referred to as xe2x80x9cnear net shapexe2x80x9d, since only the xe2x80x9cundercutxe2x80x9d or xe2x80x9cbacksweepxe2x80x9d areas need to be machined away in the subsequent machining step. Accordingly, the amount of material which must be removed by machining is minimal compared to machining a compressor wheel from a solid wrought titanium block and tool time and tool wear is negligible.
The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood, and so that the present contribution to the art can be more fully appreciated. Additional features of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other compressor wheels for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent structures do not depart from the spirit and scope of the invention as set forth in the appended claims.